- fix typos in comments
- indent code
- move the Positive module further down types.ml, so that there is no
need for forward references when we implement substitutions over record types.

 * make sure (decompose t) builds a cyclic 'v' object if t is a recursive type.
 * add a memoization environment to substitute_aux to detect and rebuild such cycles in the new 'v' object.
 * use solve everywhere to build a cycle type (this basically implements Courcelle's algorithm at the level of types.

* More agressive simplifications of unused variables, and variable beautification of remaining variable (i.e.
rename _fresh_xxx, _fresh_yyy into $A, $B, ...

Conflicts:
	types/types.ml
	types/types.mli

compile/lambda.ml

Conflicts:
	compile/compile.ml
	runtime/eval.ml
	types/types.ml
	typing/typed.ml

	implementation of sequences)

[TESTS][LAMBDA] Fix booleans; add some tests

	so they can refer to it, however a runtime error occurs when using it

Conflicts:
	types/var.ml